@UniSuperBox
So how come that it installed it on mu Pro 5?
There must a bug or something, but it downloaded and installed the Ubports image that I'm now using.
I manually flashed the recovery right before that.

I know that --download-only is intended for only downloading the files, that's what I wanted to do, but right after ubuntu-device-flash downloaded the files, it pushes them to cache and rebooted to recovery and started flash it over.
I was baffled as well.